Sam Spade unraveled the mystery of the Maltese Falcon. Now his nonfictional alter ego is snooping into a real treasure-gone-missing caper: the Maltese moon rock.

Only about the size of a pea, the dull-gray specimen was plucked from the lunar surface by Apollo astronauts, embedded in a clear acrylic ball, and given to Malta. Last month, the acrylic ball and rare rock inside it disappeared from its display in a Mdina museum.

Malta's lunar larceny fits a global trend, some experts say. In 1973, President Nixon gave nearly identical moon-rock fragments as "goodwill" gifts to 135 nations. Today nobody, including NASA, seems to know the exact whereabouts of most of those fingernail-sized samples.

And the Malta theft has reignited interest in moon rocks, observers say, as has the coming 35th anniversary of the first moon landing on July 20 - along with a sizzling market for space collectibles overall.

"I would not be surprised if half of those 135 moon rocks have been stolen, or lost, or are now in a position where they could be stolen," says Joseph Gutheinz, a former investigator for NASA's Office of Inspector General. "I've been contacting museums around the world, and I'm not having responses I would have expected. There's no real sense that any but a few of these moon rocks can be accounted for."

From con men to strongmen, college students to collectors, many have sought to buy, sell, or steal a piece of the moon. Mr. Gutheinz, who retired in 2000 from NASA,has been involved in investigating several moon-rock capers - and knows details about even more.

In 2002, three NASA interns stole a 600-pound safe with reportedly about 3.5 ounces of moon rocks inside, valued at tens of millions of dollars. The college students tried to hawk the rocks over the Internet, but were reported to authorities. All were tried and convicted, says Gutheinz, who was not on that case.

But Gutheinz did investigate the case of another goodwill moon rock that went missing from Honduras in the early 1990s. It resurfaced in 1998 while Gutheinz was running a sting operation dubbed "Operation Lunar Eclipse" to ferret out fakes. A United States citizen tried to sell a moon rock in its acrylic shell complete with plaque for $5 million. Unfortunately for him, the "buyer" turned out to be Gutheinz in his undercover role, posing as a rich collector. The rock was seized by federal agents and returned to Honduras.

That wasn't the end, though. The dealer argued in federal court he was the rightful owner, having purchased the ball for $50,000 from a retired Honduran military official. It became one of the most strangely named cases in US legal history: "United States of America vs. One Lucite Ball Containing Lunar Material."

Finally, in March 2003, a federal judge ruled that Honduras was the lawful owner, not the dealer, who was not charged. The rock made its formal return to Honduras in a February ceremony. Today, Gutheinz is on the hunt for US moon rocks given to Afghanistan, Nicaragua, Nigeria, Niger, Gabon, Republic of Congo, Romania - and Malta. He fears, though, that the Afghanistan moon rock might have been destroyed, or even sold by the Taliban regime to fund government operations or terrorism.

Lest there be any ambiguity when it comes to lunar pebbles, NASA would like to make one thing clear: You can't legally own one - not yet, anyway. In 2000, Congress introduced legislation to permit astronaut moon-rock ownership - and new moves in this direction are rumored.

For now, all the goodwill rocks belong to the countries honored by President Nixon. The rest - less than 1,000 pounds' worth - are owned by NASA. That scarcity and restriction make a moon rock like the one from Malta potentially worth millions at auction.

Con men, as well as innocent-but-ignorant sellers who think they have a moon rock, today pose another big challenge for NASA's lunar sleuths. In 1995, for instance, two brothers tried to auction a fake moon rock, claiming it was a gift from an astronaut to their late father, a former NASA worker. The brothers were reportedly caught when the auction house queried NASA and was told even astronauts can't own a moon rock.

Just a few months ago NASA confiscated a moon rock being offered on eBay which turned out to be fake, says Gary Lofgren, NASA's moon-rock curator. And recently, Dr. Lofgren was shown a sample that a major auction house had planned to place on the block last week.

"I had to tell them it was not a lunar sample," he says.

Goodwill rocks aside, the vast majority of lunar rocks are safe. NASA keeps its moon rocks under lock and key, and all but about 1 ounce of its 842 pounds of the rocks, pebbles, and dust brought back by the astronauts is accounted for, the space agency says. Dr. Lofgren's computerized inventory tracks every stone loaned out for scientific research or exhibit. What about that missing 1 ounce? It got lost in the mail, Lofgren says.

Apollo moon rocks aren't the only ones that have journeyed to Earth. Russia's three robotic lunar missions returned about 3/4 of a pound of specimens. And, in the only known legitimate moon-rock sale, Sotheby's in 1993 auctioned off three specks of Russian moon rock, weighing less than 1/100ths of an ounce, for $442,000, says Robert Pearlman, editor and founder of collectSPACE.com, a source for space-memorabilia enthusiasts.

But the Apollo moon rocks are the most desirable because of the emotional connection millions of people made when they watched on TV as astronauts collected the rocks, Mr. Pearlman says. He notes that the market for space collectibles has boosted interest in moon rocks. A piece of Apollo gravel weighing 4/100ths of an ounce would be conservatively worth $1 million, Pearlman estimates. That's roughly four times the price of a diamond that size.

Like Gutheinz, Pearlman worries that many museums don't realize the value of the moon rocks Nixon gave as gifts three decades ago. So he, too, has begun a project to find the goodwill moon rocks - tracking down just 16 so far.

Disappointed, he reports several moon rocks on display but without any serious security arrangements, while others gather dust in storage. Still others are long gone, he says. News reports in 1998, for instance, suggested that Romania's moon rock may have been auctioned off with the possessions of former dictator Nicolae Ceaucescu.

For his part, Pearlman eagerly awaits the day when large loads of moon rocks are brought back by private space ventures for individual ownership. Until then, aficionados must content themselves with tektites - small, glassy bodies found in isolated places around the world and worth about 50 cents at any rock shop. Such specimens could be from the moon - or from meteorites - but nobody knows for sure, Pearlman says.

Technically, one can buy a piece of the moon, albeit an ephemeral one. Apollo astronaut turned artist Alan Bean sprinkles lunar particles onto his paintings, culled from dust that stuck to his spacesuit. Other moon flecks have been given as NASA retirement gifts - including a 1-inch piece of transparent tape covered with moon dust, which the recipient cut up and auctioned off. Pearlman paid $5,000 for a tiny piece of moon tape - and he was not the only one.

The magic of moon particles seems to act like pixie dust on collectors. "If I can't visit the moon, at least I can hold a piece of it in my hand," Pearlman says. "My ultimate hope is that someday I will be able to own a moon rock myself."

Until then, as Sam Spade might say, it appears that moon rocks remain the stuff that dreams are made of.

• Intense heat and a notable absence of water forged the three types of moon rocks: basalts (lava rocks), anorthosites (light rocks), and breccias (composite rocks). Unlike soggy Earth, the moon has no sandstones, shales, or limestones.

• On its last manned flight to the moon, NASA finally sent a true rock hunter - geologist Harrison Schmitt. Unsurprisingly, Apollo 17 brought back more rock samples than any other mission to the moon.

• Were it legal to sell one, a one-gram moon pebble (less than 4/100ths of an ounce) would fetch $1 million or more - while a top-quality diamond of equal size commands a relatively modest $250,000.